I'm really liking the progress you're making on the halfling, looking forward to seeing it finished. I have trouble with the armpit too, I usually have to stare at my own for longer that is really comfortable. A book that's really helped me with learning figure drawing is by Michael Hampton just called Figure Drawing, I've gone through it a couple of times

Alright, so I'm still learning anatomy too, but I'd say the major thing right now that doesn't look right to me are the pectoral muscles.

When lifting your arm like that and from that point of view, the pectoral muscles will move over the the collar bone and wrap around behind the arm. This will also hide a big part of the deltoid and even a part of the pec itself. I tried to show what I mean below using a basic diagram that I hope will be clear enough.

I also think that you can bring the lower part of the abdominal muscles lower towards the crotch. And I think there should be a bit of the lateral muscles showing, especially on his left side (our right). But I'm not confident enough to attempt drawing them unfortunately. But they would come from behind and wrap around to connect somewhere around the armpit I believe.

So I hope this helps and if you have any other questions feel free to ask. I'll try my best to answer